Natural gas is a combustible, gaseous mixture of several different hydrocarbon compounds and is typically extracted from deep underground reservoirs formed by porous rock. The composition of natural gas extracted from different reservoirs varies depending on the geographic location of the reservoir. In fact, it is not entirely uncommon for the composition of gas extracted from a single given reservoir to vary to an extent. Regardless of any variations, however, the primary component of natural gas is methane, a colorless, odorless, gaseous saturated hydrocarbon. Methane usually makes up from 80% to 95% of any natural gas sample and the balance is composed of varying amounts of ethane, propane, butane, pentane and other hydrocarbon compounds.
Natural gas is used extensively in residential, commercial and industrial applications. It is the dominant energy used for home heating with well over half of American homes using natural gas. The use of natural gas is also rapidly increasing in electric power generation and cooling, and as a transportation fuel.
Natural gas, like other forms of heat energy, is measured in British thermal units or Btu. One Btu is equivalent to the heat needed to raise the temperature of one pound of water by one degree Fahrenheit at atmosphere pressure.
A cubic foot of natural gas has about 1,027 Btu. Natural gas is normally sold from the wellhead, i.e., the point at which the gas is extracted from the earth, to purchasers in standard volume measurements of thousands of cubic feet (Mcf). However, consumer bills are usually measured in heat content or therms. One therm is a unit of heating equal to 100,000 Btu.
Three separate and often independent segments of the natural gas industry are involved in delivering natural gas from the wellhead to the consumer. Production companies explore, drill and extract natural gas from the ground; transmission companies operate the pipelines that connect the gas fields to major consuming areas; and distribution companies are the local utilities that deliver natural gas to the customer.
In the United States alone, natural gas is delivered to close to 200 million consumers through a network of underground pipes that extends over a million miles. To produce and deliver this natural gas there are over a quarter-million producing natural gas wells, over one hundred natural gas pipeline companies and more than a thousand local distribution companies (LDCs) that provide gas service to all 50 states.
Prior to regulatory reform, which essentially restructured the industry, producers sold gas to the pipeline companies, who sold it to the LDCs, who sold it to residential and other customers. Post-regulation, however, pipeline companies no longer purchase gas for resale. Instead, the pipeline companies merely transport gas from sellers, such as producers or marketers, to buyers, such as electric utilities, factories and LDCs. Thus, the LDCs now can choose among a variety of sellers of natural gas, whereas before they could only buy gas from one source, i.e., the pipeline company. Further, some states have implemented additional restructuring which renders the LDCs subject to regulation by State public utility commissions. Prior to these additional state regulations, an LDC's residential customers could only buy gas from one source, i.e., the LDC. After state regulation, however, residential customers can choose a different supplier other than their LDC from which to buy the gas. The consumer's LDC, as the owner/operator of the distribution network, delivers the gas to the consumer, as before, but the LDC only charges the consumer for delivery of the gas and the independent supplier bills for the gas.
As a result, sampling and analysis of the natural gas along various points in the pipeline network has become an increasingly more important endeavour. More particularly, because consumers are typically billed for natural gas in Btu's, it is important that the Btu measurement of any particular gas volume be accurate. Further, because various suppliers can, and do, supply their respective gas, which comes from widely varying geographic locations, to the single network of pipelines, the measured Btu value within a given section of pipe will vary.
According to the current state of the art in gas sample conditioning, gas samples are extracted via a probe from a gas pipeline by using a so-called insertion probe. Once the gas sample is extracted, it is typically provided through stainless steel tubing with a relatively small diameter to an analyzer, such a, a chromatograph, for analysis. A chromatograph is a device that utilizes a family of analytical chemistry techniques to separate mixtures into their constituent components. Typically, the techniques utilized by a chromatograph include separating the components of the mixture on the basis of differences in their affinity for a stationary and a mobile phase.
The distance between the gas line and the analyzer often exceeds thirty (30) feet and may even exceed one-hundred (100) feet. Across this length, the gas sample moves from a zone of high pressure at the probe, e.g., 2000 psig, to a relatively low pressure zone, e.g., 10–30 psig, the preferred pressure for a typical analyzer/chromatograph. Due to this rather substantial decrease in pressure, also known as adiabatic compression, the gas sample is cooled, toward its freezing point. If the gas sample temperature decreases below the gas dew point, condensation occurs. This phenomenon is known as hydrocarbon dew point dropout.
When this happens, the analyzer/chromatograph can potentially be seriously damaged because chromatograph devices ideally operate on dry input gases. When a chromatograph is damaged in this manner it must be taken offline in order to perform the repair. This downtime results in higher costs and inaccurate measurements. Accordingly, it is increasingly more important to maintain the gas being sampled at a constant temperature in order to reduce the chances of hydrocarbon dew point dropout in the sampled gas.
The issue of hydrocarbon dew point dropout in gas sampling has been addressed by heating the sample gases, as well as the pressure regulators, gas lines, and other components, that come into contact with the sample gas, between the pipeline and the analyzer/chromatograph to keep the temperature of the gas above its dew point, thus preventing any of the gas being sampled from entering its liquid.
Natural gas sampling systems, however, are typically located in harsh environments, e.g., where outdoor ambient temperatures can be significantly below the gas dew point temperature and where dangerous explosion-prone gas vapors are often permeating into the surrounding atmosphere. Accordingly, the heating mechanism used must adhere to strict standards in order to generate enough heat to overcome the low ambient temperature and do so without exposing the dangerous hydrocarbon gases in the atmosphere to any safety problems, such as electrical wiring, etc.
For example, the American Petroleum Institute (API) has suggested heating the sample probe and using heat tracing lines to heat the gas line between the probe and the analyzer. Such systems often rely on catalytic heaters to maintain temperature stability and avoid undesirable temperature changes to the gas sample communicated between a source, e.g., pipeline and the analyzer. Catalytic heaters of the type referred to by the API in its Manual of Petroleum Standards call for heating a sample gas stream throughout a selected portion of a system where the heated sample is then introduced into a heated tube bundle of the analyzer.
Several problems are known to exist with prior art gas sample heating systems. For example, in the case where a heater system employs electrical initiation, in order to ignite the heaters when they are used in the field, these related art systems typically rely on the use of relatively large batteries, (e.g., 12V batteries similar to those used in an automobile). Obviously, exposing the hydrocarbon vapors in and around the sampling system to a spark potentially created by 12 volt car battery is not a desirable situation, e.g., due to the high risk of explosion.
Other related art systems, e.g., catalytic heaters adapted to operate by burning stored natural gas, are not economical and suffer from an increased operating expense, particularly under present market conditions. Another, perhaps more significant, issue with conventional sample heater systems is the failure rate of such systems. Because failure rates of typical conventional systems are at unacceptably high levels, the reliability of the sample testing systems have become problematic.
Some systems have been developed in an attempt to address the foregoing concerns regarding conventional gas sampling systems. For example, PGI International, of Houston, Tex., has produced a gas-sampler called the “Hot-Shot” that features a catalytic heater enclosed in an insulated stainless steel cabinet. The catalytic heater in the “Hot-Shot” utilizes infrared heat to maintain the gas sampling components at a temperature between 100 degrees and 140 degrees F.
Another known system, the High Pressure Gas Sampling System, Model 2020, produced by Tekran Inc., of Toronto, Canada, is illustrated in FIG. 7. The Tekran system includes an electrically heated pressure regulator for eliminating, sample condensation during pressure reduction. As shown, the system of FIG. 9 accepts natural gas at an inlet pressure of 50–2000 PSI before passing the gas through valves, vents, the pressure regulator, gauges and a manual injection port, before releasing the gas at an adjustable outlet pressure of between 0 and 30 PSI. The Tekran system, however, requires that an external 110–125 volt A.C. power supply be provided to provide heat to the heated regulator.
Another known system is described in U.S. Pat. No. 5,611,846, to Overton et al. Overton et al. discloses a portable gas chromatograph that operates at 100 watts peak power, 12 V DC, supplied to the unit from a power pack or other DC source; the power pack supplies 140 watt-hrs of uninterrupted power at 12 V DC.
Other conventional systems not necessarily associated with gas sampling in hazardous environments have also been suggested. For example, certain systems related to the medical arts describe the use of heated pressure regulators for controlling pressure, flow rate, and temperature of a gas being administered to a patient. There exists, however, a need for improvement to the presently accepted and commonly used systems and methods of heating gas samples in the field.